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Oak Ridge Poster Sessions |
Clin. Labs., Onassis Cardiac Surgery Center, 356 Syngrou Ave., GR-176 74 Kallithea, Greece
a author for correspondence: fax + 301 93 06 604, e-mail mdb1{at}hol.gr
Heparin is administered as an anticoagulant in treatment of or prophylaxis against arterial or venous thromboembolism, in treatment of myocardial infarction, and during cardiac surgery (cardiopulmonary bypass) (1), coronary angioplasty, and other procedures (2).
Heparin treatment demands laboratory control and is monitored by determining the activated partial thromboplastin time (APTT), which should be adjusted to stay within 1.5–2 times the control value (3). Sometimes during therapy high APTT values (>180 s) are reached (4), and physicians decide to interrupt the treatment for a few hours because of the great risk of hemorrhagic complications, the most important side effect of heparin therapy (5). In these situations, however, the true concentrations of heparin in the patient's plasma are really unknown. Moreover, in some described cases, the APTT values were prolonged because of underlying disease (severe liver disorders, extensive myocardial infarction, or infection) and postoperative complications after cardiac surgery (6). This prolongation of APTT was not heparin related, although the results of the APTT test falsely suggested a higher heparin concentration (7). Another problem is that sensitivity of the reagents for the APTT test varies greatly, and the physicians who prescribe heparin on the basis of certain APTT ratios may order different doses of heparin and produce different amounts of anticoagulation in their patients (3).
In cases with the patient's APTT >180 s, the use of a plasma heparin assay may in fact be better for anticoagulation control. That is, measurement of anti-Xa activity of heparin in plasma may be more specific for monitoring therapy and be superior to APTT determination (8)(9). In the present study, we investigated the correlation between heparin activity in plasma (anti-Xa activity) and the incidence of bleeding complications in patients with APTT >180 s.
The procedures used were in accordance with the Helsinki Declaration of 1975, as revised in 1983. All 47 patients in our study (ages 62 ± 8.9 years, range 43–77) were treated with a continuous intravenous infusion of heparin for management of thromboembolic disorders or as a prophylaxis against any (Heparin Leo; Leo Pharmaceutical, Ballerup, Denmark; 2500–5000 IU as a bolus dose and 1000 IU/h after).
In detail, four patients had acute myocardial infraction, five had unstable angina, one had rheumatic mitral valve disease, one had pulmonary embolism, and one had atrial fibrillation. Twenty-seven patients had undergone percutaneous transluminal coronary angioplasty, four cardiac catheterization, three coronary bypass surgery, and one valvuloplasty. Four of the 47 patients had thrombocytopenia at the time of the study. No patient had malignant or hematological disease or stroke, and all had normal renal function. These patients were not receiving any drug, besides heparin, that might alter the results of the anti-Xa activity or the APTT assay.
The APTT values of the patients were >180 s when the plasma samples of this study were collected. Any major and minor bleedings that happened during that time of heparin therapy were recorded. Bleedings were defined as major that led to a blood transfusion, to a decrease in hemoglobin by >24 g/L, or to formation of surgically treated hematomas; all other bleedings were defined as minor.
The plasma samples were collected into sodium citrate as anticoagulant (10:1 by vol. final concentration) with use of the Vacutainer Tube system (Hemogard tubes no. 367714; Becton Dickinson) and were separated by centrifugation (at ambient temperature, 10 min, ~1500g) within 10 min from collection. They were stored at -70 °C until assayed.
The APTT assay (CTS Neothromtin; Behring) was performed in a semiautomated instrument (Behring Chromotimer) with a chromogenic substrate. According to this method the patient's plasma (50 µL) is incubated with optimal amounts of phospholipids and an activator (25 µL). The result is the activation of the factors of the intrinsic pathway. Coagulation is then triggered by the addition of calcium ions. Absorbance at 405 nm increases with the formation of activated thrombin, and the result is determined on the basis of the time taken for this absorbance to increase by 0.1. The reference interval, according to the manufacturer, is 28 to 40 s.
To determine anti-Xa activity of unfractionated heparin (expressed in IU/mL), we performed the Berichrom Heparin assay (Behring) in the same instrument. In the incubation phase of this method, antithrombin III inactivates factor Xa in a reaction catalyzed by heparin. Dextran sulfate releases whatever heparin has bound to interfering factors and thus makes it accessible to the assay. The quantity of factor Xa remaining after the incubation is determined via the increase in absorbance at 405 nm, with use of a chromogenic substrate in a kinetic test. This sensitive assay is not disturbed from the possible abnormal concentrations of fibrinogen or fibrin/fibrinogen degradation products.
The assay was calibrated in the range 0–1 IU/mL (0–1000 IU/L) by comparison with known plasma concentrations of heparin (Heparin Leo 5000 IU/mL, i.e., 5 x 106 IU/L in Behring's human plasma calibrator). Concentrations >1 IU/mL were determined after diluting the plasma with an equal volume of isotonic saline.
All 47 patients treated with heparin had APTT values >180 s. Their mean anti-Xa activity (±SD) was 0.94 ± 0.21 IU/mL, within a range of 0.54–1.29 IU/mL, and 8 patients had anti-Xa values within the therapeutic range of 0.35–0.7 IU/mL. In 18 patients (38.3%), the heparin treatment led to bleeding complications, 11 minor (23%) and 7 major (15%).
Patients were then categorized in two groups: the 26 whose anti-Xa
activity was <1 IU/mL and the 21 with anti-Xa activity >1 IU/mL
(>1000 IU/L). The risk of bleeding in these two groups was found to be
23% and 57%, respectively (Table 1
). The difference between incidences of bleeding (%) in the
groups was statistically significant (P = 0.037, Yates
corrected 
2 test). Major bleeding
complications were seen in 3 of the 26 patients (11.5%) with anti-Xa
activity <1 IU/mL and in 4 of the 21 patients (19.0%) with anti-Xa
activity >1 IU/mL (not significantly different).
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Our results indicate that the incidence of bleeding risk was significantly increased in patients with anti-Xa activity >1 IU/mL. Given that in patients with APTT >180 s the exact degree of anticoagulation is unknown, anti-Xa activity may be clinically informative and can contribute to the control of bleeding complications. Moreover, interruption of treatment in patients with APTT >180 s, who have therapeutic concentrations of plasma heparin, leads to inadequate protection against thromboembolic disease.
References
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